Structural Changes in Chromosomes 111 



12.6. In human chromosomes at mitotic metaphase, discuss the detectability of the 

 following: 



(a) Paracentric inversion (c) Deficiency 



(b) Pericentric inversion (d) Duplication 



(e) Half-translocation 



12.7. What advantages may inversion provide? 



12.8. What characteristics of cells undergoing oogenesis favor the production and 

 viable transmission of half-translocations? 



12.9. In Drosophila, a male, dihybrid for the mutants bw and ,s7, when back-crossed 

 to bw bw st st, normally produces offspring whose phenotypes are in a 1:1:1:1 

 ratio. On exceptional occasions, this cross produces offspring having only two 

 of the four phenotypes normally obtained. How can you explain such an 

 exception? 



12.10. Is the telomere a gene? Why? 



12.11. Explain how you could cytologically determine the position of the locus for 

 white on the X chromosome of Drosophila by each of the following: 



(a) deficiencies of various sizes 



(b) inversions of various sizes 



(c) various reciprocal translocations 



12.12. Suppose you had a self-maintaining strain of Drosophila in which all females 

 were yellow-bodied and males, grey-bodied. How would you explain this con- 

 sistency if the egg mortality were always 50%? Low, as it is normally? How 

 would you test your hypothesis cytologically? 



12.13. (a) Several X-linked mutants in Drosophila cause notched wings. One of 

 these mutants is lethal in the male and also in the mutant homozygote female. 

 How do you suppose such a homozygote is produced? 



(b) A female heterozygous for this mutant (N/+) is mated to a fa/Y male. 

 In F x all sons are normal, half the daughters are normal, and half are both 

 notched and faceted. Explain this result showing how you might test your 

 hypothesis. 



12.14. Make a diagram of the different eucentric reciprocal translocations between 

 autosomes 2 and 3 in Drosophila which you would expect to be lethal in the 

 following cases: 



(a) when either half-translocation is present 



(b) when one half-translocation but not the other is present 



(c) under no circumstances 



12.15. Does the absence of crossing over in male Drosophila facilitate the detection 

 of heterozygous reciprocal translocations? Explain. 



12.16. Given a Drosophila heterozygous for a eucentric reciprocal translocation be- 

 tween chromosomes 2 and 3 and assume both half-translocations are lethal 

 when present separately. Discuss the nature of the linkage maps one would 

 obtain from mating 



(a) genetically marked females of this type with appropriately marked non- 

 translocation males 



(b) genetically marked males of this type mated to appropriately marked non- 

 translocation females 



12.17. A chromosome A.BCDEEDCFG has a reverse repeat, or duplication, for CDE. 

 Compare the stability of this chromosome with A.BCDECDEFG, which carries 

 a tandem repeat, or duplication, for the same region. 



